1. Field of the Invention
The present invention relates to an image processing method and system in which representation of the difference between two or more images taken of the same subject is improved.
2. Description of the Related Art
It is a common practice in a wide variety of fields to read and compare a plurality of two or more images taken of the same subject in order to discern the difference between images, and carry out an inspection of the subject based on the thus discerned difference, etc.
In the industrial products manufacturing industry, for example, images taken of a product when it is new and images taken of the same product after it has been subjected to an endurance test are compared, and attention is focused on the area showing the biggest difference so that an examination as to whether or not there are areas requiring improvement in product endurance can be carried out. In the medical field, doctors read and compare a plurality of radiation images of the diseased part of a patient taken in a time series manner in order to ascertain the course of the disease and determine the proper treatment.
In this fashion, reading and comparison of a plurality of images is carried out daily in a wide variety of fields, with the images to be read and compared being output to a display means such as a monitor, to film, or another recording medium. In other words, the image is converted to luminescence and density signals and displayed on a monitor, etc., or output to film or another hard-copy recording medium by a printer or the like.
Generally, although the area of the most interest to the viewer of such images is the area wherein the greatest difference is shown, when the images to be read and compared are output, they are simply lined up and output. However, under such conditions, the smaller the difference between the images is, the more difficult it becomes to discern.
Therefore, image processing methods that facilitate discernment of the difference between images with a higher degree of accuracy and reliability are desired. These generally start with a subtraction process that calculates the difference between corresponding structural positions (anatomically characteristic positions) of the two images, followed by extraction and enhancement of the aforementioned difference (Japanese Patent Application No. 11(1999)-342900). In this way, only the difference between images is extracted and enhanced, thereby enabling the viewer to ascertain the difference with certainty, and this is considered to contribute greatly to the prevention of misdiagnosis due to oversight of a diseased part requiring treatment or the course of progression of a disease.
In addition, when computation of the interimage-difference is to be performed, it is necessary to first align the corresponding positions of structural bodies appearing in each image (structural positions). Such alignment might entail as a first alignment, for instance, a global transformation (affine transformation, etc.) of parallel displacement, rotating and enlarging or reducing the images, and the image obtained thereby is then subjected to a second alignment entailing the division of the aligned image into a plurality of local-regions, and then matching each corresponding local-region, based upon which a non-linear distortion translation (warping) dependent on curve-fitting (two-dimensional, 10th order polynomial function) is performed. The utilization and combination of such techniques, etc. are known as shown in Japanese Unexamined Patent Publication No. 7(1995)-37074, etc.
However, after aforementioned alignment has been performed, small misregistrations occur between the outlines of structural positions that have not been perfectly aligned (position-misregistration), and artifacts appear in an interimage-difference image where position-misregistrations occur at such outlines. Up until now, if no artifacts appeared in an interimage-difference image, because only the substantial difference between two images, representing the change due to the progress of the course of a disease, etc. of a diseased part, showed up, the size of a diseased part, the course of a disease, etc. could be effectively judged. If artifacts appeared in the image however, they obscured and thereby prevented the substantial difference from being discerned with accuracy, creating the possibility that the difference might be overlooked.
Particularly for cases in which the images are radiation images, because the three-dimensional (3-D) misalignments, occurring due to the slight changes in the direction from which the radiation is emitted, the axis of the body and its angle, the angle at which the radiation enters the subject, etc., represented in a two-dimensional (2-D) image are exceedingly difficult to correct by use of alignment techniques, the appearance of artifacts is to some extent unavoidable.
However, it is necessary to eliminate the potential that the substantial difference be overlooked due to the appearance of artifacts.